Piezoelectric devices are used in a wide variety of applications, notably as actuators and sensors. In general terms, when an electrical potential is applied to piezoelectric material, the material undergoes a conformational change. This phenomenon is leveraged in a piezoelectric actuator to enable electronic control over the position of a component coupled with a piezoelectric element of the actuator. One common application for piezoelectric actuators is in the fuel injection arts, wherein piezoelectric actuators are used to control the position of a valve. Piezoelectric actuators offer a variety of advantages over other known actuator systems. For example, piezoelectric actuators tend to be relatively easy to precisely and accurately control, and are typically relatively robust. In the context of fuel injectors, piezoelectric actuators may be required to actuate millions or even billions of times over the course of a fuel injector's service life.
A downside to piezoelectric actuators relates to the relative precision typically desirable in preloading the piezoelectric elements used therein. In other words, a particular mechanical bias is usually required to enable a piezoelectric element to reliably and appropriately change its conformation in response to an electrical potential. Piezoelectric materials, such as certain crystalline and ceramic materials, will typically exist in a low energy state when no preload and no electrical potential is applied thereto. Application of an electrical potential to a piezoelectric element which is not sufficiently preloaded can cause it to break. Conversely, where piezoelectric elements are excessively preloaded, they may not be able to successfully actuate when an electrical potential is applied thereto. Improper preloading can also reduce the service life of the piezoelectric element. Engineers have heretofore found it relatively difficult to set piezoelectric element preloads at just the right level.
One attempt at preloading piezoelectric actuators is known from U.S. Pat. No. 6,998,761 B1 to Frank et al. In the strategy describe et al., a piezoelectric actuator is pressed into a hollow body with a defined force by an assembly device to elongate the hollow body. The defined force purportedly corresponds to desired prestress of the actuator. While maintaining the force, a cover plate is welded to the hollow body to fix the prestress of the actuator. While potentially applicable in certain instances, the approach of Frank et al. is unlikely to achieve sufficiently precise preloads to provide optimum actuator performance and durability. The purported “defined force” applied by a mechanized force device will tend to be inherently variable. In particular, tolerance issues with regard to the mechanized force device, as well as tolerance issues relating to the actuator itself may result in actuators being improperly prestressed.